Prior to the present invention, minute volume has been typically measured using three electrodes, with two of the electrodes being in the same cardiac chamber (i.e., either the atrium or the ventricle), and the third electrode being at the end of a lead or comprising the pacemaker case. A current was passed between one of the electrodes in the cardiac chamber and the third electrode. The resulting voltage between the other electrode in the cardiac chamber and the third electrode was measured. The current and the voltage values were then used to determine the transthoracic impedance dynamically. (See, for instance, U.S. Pat. No. 4,702,253.) A problem with this approach is that it requires at least two electrodes disposed within the heart. As a result, MV type pacemakers could not be implanted to replace old pacemakers using a unipolar lead (with only one electrode), or a bipolar lead one of whose wires is broken (thus also having only one effective electrode).
U.S. Pat. No. 4,901,725 shows how to determine minute volume using standard bipolar leads, each bipolar lead consisting of a ring and a tip electrode. In this patent, current is injected between the ring electrode and the pacemaker case. The voltage between the tip electrode and the case is used to determine the transthoracic impedance.
U.S. Pat. No. 5,562,712 teaches how to measure MV using two unipolar leads, the two leads terminating at electrodes in different cardiac chambers. This configuration is useful when an MV pacemaker replaces a standard pacemaker coupled to two unipolar leads, rather than a bipolar lead.
Of course, a major disadvantage of all these arrangements is that they still require at least two electrodes in the heart of the patient. In the prior art, an MV pacemaker could not be used to replace a pacemaker used with a unipolar lead, or used with a multi-wire lead in which only one wire is still intact.